Implement a set of reusable Google Sheets libraries for internal support operations

[annagav]

The original implementaiton of Google Sheets was done in xPro. Both MITx Online and xPro are going to use Google Sheets but the former will only use parts of the functionality currently present in xPro.

The xPro implementation was mainly focused around bulk enrollments and coupon assignment functionality, both of which are not going to used by MITx Online. The implementation of Google Sheets has tight coupling between all the different aspects of implementation so it a) does not make for an easy targeted extraction of the desired functionality and b) reflects that the implementation needs substantial refactoring to avoid these design issues.

After discussion with Nathan we decided to put google sheets in ol-django, separating it into multiple libraries:

In the short term:

• #68
• #69

At some point in the future:

• Implement a library that's functionally equivalent to xPro's bulk enrollments
• Implement a library that's functionally equivalent to xPro's coupon assignments